Last month we explored how plants absorb water from the soil and return it to the air in a process called evapotranspiration. This month we will see how agricultural scientists at the University of Arizona and elsewhere have developed ways of predicting exactly how much water is used by plants so that farmers and gardeners like us can know exactly how much water to replace that which is lost.

These agricultural scientists grow turn grass on huge but very sensitive scales called lysimeters so that they can tell exactly how much water is applied to the turf and how much is lost through evapotranspiration. By carefully measuring such external factors as temperature, solar radiation, humidity, and wind speed, they build models that predict how much water the grass will use. In scientific parlance, a model is a replica of something a scientist wants to study. The exact type of model depends upon the purpose of the study. The exact type of model depends upon the purpose of the study. Models may be small physical representations of an object (iconographic models). Descriptions of an object that reside inside a computer (computer models), or mathematical equations that describe the behavior of something (mathematical models). The model used by scientists at the University of Arizona to predict evapotranspiration is a mathematical model called the Penfield Equation which was developed by scientists for the California Irrigation Management Information System (CIMIS). The value calculated using this equation is called potential evapotranspiration (ETo).

Knowing ETo helps nurserymen, farmers, greens keepers, and other professional growers apply just the right amount of water required to grow healthy plants. The Arizona Meteorological Network (AZMET) has been supplying these professionals with ETo data since 1986. Recently they began putting these data on the World Wide Web so that anyone with access to the internet can take advantage of this information. The URL for AZMET is:

ag.arizona. edu/AZMET. To provide this service, AZMET maintains 23 field sites at various locations throughout Arizona where meteorological data are collected. Each of these sites is equipped with a sophisticated suite of instruments to measure data on air and soil temperatures, relative humidity, solar radiation, precipitation, wind speed, and wind direction which are reported hourly. ETo is calculated from these data and also reported hourly. As might be expected, these reporting stations are concentrated central Arizona in areas with heavy agricultural development. The closest station to us in Cochise County is at a site called Bonita 18 miles north of Willcox in Graham County at an altitude of 4416 feet. The soil type at the Bonita station is a loamy sand (77% sand, 16% silt, and 7% clay).

You can view hourly, daily, weekly, and monthly data for the Bonita station from 1987 to the present in two formats. Raw data files are in comma delimited format suitable for direct importation to a spreadsheet or database. Reports contain the same data formatted in a more human- friendly way. Monthly reports summarize the data by day and are probably the most useful for gardeners. Table 1 on Page 5 shows data for July through the 14th.

The table shows us that the total ETo for July as of the 14th at Bonita was 3.72 inches. This means that - all other things being equal - 3.72 inches of water would have to be applied to an area to replace the water that was lost to evapotranspiration. To translate that into something you can more easily relate to your water bill, 3.72 inches of water covering an area of one square foot equates to about 2.3 gallons. An area of turf measuring 25 X 50 feet would require 2,875 gallons of water to replace what was lost to evapotranspiration! Now you know why we advocate xeriscaping with minimum turf. Next month I will discuss how you can use ETo data to estimate how much water is required to maintain your garden. In the meantime... HAPPY SURFING.